In the prior art, volumetric computed tomography (VCT) systems are conventionally operated by rotating the source and detector around a rotational axis (often called the z-axis), thereby obtaining image measurements from multiple angles. These measurements are then computationally combined to create a three-dimensional representation of an object within the field of view of the system. Many such systems have a single x-ray source that travels in a circular path as the system rotates. It is known that such systems suffer from “cone-beam” artifacts. To avoid these artifacts, some VCT systems use a source with an array of source elements separated in the axial direction. The length of the source and detector in the axial direction defines an axial extent of a VCT system. If the source and detector movement is limited to rotation, then the axial extent corresponds to the axial field of view, i.e., the total thickness of the volume acquired by the system during an entire rotation. However, the axial field of view of a CT system can be made larger than the axial extent by introducing a translation of the source and detector during the rotation, e.g., as in helical scanning methods. In this mode, the total z-translation during a complete rotation is typically comparable to or larger than the axial extent of the system. Thus, helical systems have been designed to perform relatively large translations to provide larger axial field of view. One exception is cardiac scanning, in which case the helical pitch is reduced so as to acquire temporal data of the cardiac motion.
VCT systems that use an array of source elements suffer from a sampling problem that derives from the fact that the source elements have discrete axial positions. In highly collimated systems, there may be gaps in the sampling. Axial planes corresponding to source element rows have both in-plane projection measurements (i.e., measurements where the source and detector are both in the same axial plane) and cross-plane projection measurements (i.e., measurements where the source and detector are in different axial planes), while axial planes between source element rows only have cross-plane projection measurements. Thus, intermediate planes between the source planes have different imaging characteristics than the source planes. In other words, due to the discrete z-positions of the source element rows, there is a z-dependence of the impulse response of the system. If the impulse response is z-dependent, the image quality of the reconstructed volume varies depending on the z-location. More specifically, the in-plane rays are necessary for sufficient sampling of the axial planes. Therefore, the axial planes corresponding to source element rows will be reconstructed more accurately than the planes between source element rows.